The United States and most other developed countries currently rely heavily on oil, coal, and natural gas for their energy needs. As supplies of these fossil fuels diminish, and as concerns regarding pollution and global warming increase, there is a growing need for alternate sources of energy. Nuclear power generation offers an alternative to the burning of fossil fuels to produce electricity.
The U.S. nuclear power industry presently supplies approximately 20% of the nation's electricity. The industry achieved its third straight year of record power generation levels during the year 2000, which represents continued growth in power production for technology that had been used to produce only 577.0 billion kilowatt-hours as recently as 1990. (See the Web site having URL www.eia.doe.gov/neic/infosheets/nuclear.htm for further information.) As of the year 2000, there were 66 nuclear power plants (composed of 104 licensed nuclear reactors) in the U.S. Other nations that use nuclear power to supply a significant portion of their electricity needs include France (57 reactors), Japan (53 reactors), the United Kingdom (33 reactors), and Russia (30 reactors). Thus nuclear power generation plays an important role in supplying electricity in the U.S. and throughout the world.
In general, a nuclear power plant operates in a similar fashion to a fossil fuel plant, with one major difference: the source of heat. The production of energy in a nuclear power plant is achieved by the fissioning or splitting of uranium atoms, which releases energy in the form of heat. Uranium found in nature consists largely of two isotopes, U-238 and the much less abundant U-235, which constitutes only approximately 0.7% of naturally occurring uranium. Most of the commercial nuclear power reactors operating or under construction in the world today require uranium enriched in the U-235 isotope.
U-235 and U-238 are chemically identical, but differ in their physical properties, in particular their mass. This mass difference makes it possible to separate the isotopes and thus to increase, or enrich, the percentage of U-235 in a given sample of material. Most reactors use enriched uranium in which the proportion of U-235 has been increased to approximately 3-4%. Although a variety of enrichment processes have been demonstrated on a laboratory or prototype scale, only two, the gaseous diffusion process and the centrifuge process, are operating on a commercial basis. Both of these processes require the conversion of uranium (which typically leaves the mine as a stable oxide known as U3O8), into uranium hexafluoride (UF6). The gaseous diffusion process involves forcing UF6 gas at high pressure through a series of porous membranes, which results in a separation of molecules containing the lighter U-235 atoms from molecules containing the heavier U-238. In the centrifuge process UF6 gas is spun at high speed in a cylinder(s) under vacuum. Heavier molecules containing U-238 increase in concentration towards the cylinder's outer edge while lighter molecules increase in concentration towards the center.
The enrichment processes described above suffer from a number of drawbacks. UF6 is a hazardous compound, which reacts with water to form the highly corrosive hydrogen fluoride (HF). The production of UF6 is further encumbered due to the required use of HF or F2. If released into the atmosphere, gaseous UF6 combines with humidity to form a cloud of particulate UO2F2 and HF fumes. In order to transport UF6 it is typically converted into the solid form, which requires a low pressure. Rapid expansion, with the potential for rupture, can take place if there are rapid increases in temperature or pressure.
Enrichment processes can also consume a significant amount of energy and generate deple UF6 as a waste product. Most depleted UF6 produced so far is stored in steel cylinders in yards near the enrichment plants, where they are subject to corrosion. The integrity of the cylinders must therefore be monitored and maintained, which requires handling. The painting must be refreshed from time to time. This maintenance work requires moving of the cylinders, causing further hazards from breaching of corroded cylinders, and from handling errors. In addition, equipment such as high-speed centrifuges needed for enrichment processes is expensive and must be constructed to meet exacting specifications. As a consequence of the many disadvantages of currently available enrichment technologies, there is a need for new methods or uranium enrichment. In addition, there is a need for improved methods of separating isotopes for purposes of treating enriched uranium stocks and for the purification of uranium (and other radioactive compounds) from waste mixtures.